1. Field of the Invention
The present invention relates to amplifiers, and is particularly concerned with microphone amplifiers which may be used in telephone systems.
2. Description of the Prior Art
It is known in the art to use microphone amplifiers in telephone systems, and in fact when, for example, piezoelectric microphones are provided. Various circuit structures for these microphone amplifiers are also already known in the art. In microphone amplifiers of this type, the output signal is superimposed upon the feed current (see Siemens Datenbuch 1974/75, Vol. 2, "Lineare Schaltungen", pp. 213-215; and German Pat. Nos. 17 62 924 and 18 08 247). These microphone amplifiers comprise, by way of an input stage, a differential amplifier comprising two transistors provided with resistance stabilization in the emitter circuit, by way of an output stage a Darlington amplifier which operates as a controlled voltage source, a negative voltage feedback network connected between the output and the input, and a reference voltage source which serves to stabilize the operating point of the differential amplifier.